Vacuum pumps having multiple inlets are well known in the art. An example of such a pump, configured as a turbo-molecular pump, is described in U.S. Pat. No. 6,709,228. These types of pumps are suitable for differential pumping multiple chambers, amongst other applications.
In a differentially pumped mass spectrometer system a sample and carrier gas are introduced to a mass analyser for analysis. Typically, the sample is ionised and the carrier gas has neutral charge. An example of such a mass spectrometer is shown in FIG. 1. With reference to FIG. 1, in such a system there exists a high vacuum chamber 10 immediately following first and second evacuated interface chambers 12, 14. The first interface chamber 12 is the highest-pressure chamber in the evacuated spectrometer system and may contain an orifice or capillary through which sample ions are drawn from an ion source into the first interface chamber 12, and ion optics for guiding ions from the ion source into the second interface chamber 14. The second, middle chamber 14 may include additional ion optics for guiding ions from the first interface chamber 12 into the high vacuum chamber 10. In this example, in use, the first interface chamber is at a pressure of around 1 mbar, the second interface chamber is at a pressure of around 10−3 mbar, and the high vacuum chamber is at a pressure of around 10−5 mbar. The unionised carrier gas is removed from the mass spectrometer chambers by the vacuum pump
Both the high vacuum chamber 10 and second interface chamber 14 are evacuated by means of a compound vacuum pump 16 having multiple inlets. In this example, the vacuum pump has two pumping sections in the form of two sets 18, 20 of turbo-molecular stages, and a third pumping section in the form of a Holweck drag mechanism 22; an alternative form of drag mechanism, such as a Siegbahn or Gaede mechanism, could be used instead. Each set 18, 20 of turbo-molecular stages comprises a number of rotor 19a, 21a and stator 19b, 21b blade pairs (three are shown in FIG. 1, although any suitable number could be provided) of known angled construction. The Holweck mechanism 22 includes a number of rotating cylinders 23a (two are shown in FIG. 1 although any suitable number could be provided) and corresponding annular stators 23b and helical channels in a manner known per se.
In this example, a first pump inlet 24 is connected to the high vacuum chamber 10, and fluid (or gas molecules) pumped through the inlet 24 passes through both sets 18, 20 of turbo-molecular stages in sequence and the Holweck mechanism 22 and exits the pump via outlet 30. A second pump inlet 26 is connected to the second interface chamber 14, and fluid pumped through the inlet 26 passes through set 20 of turbo-molecular stages and the Holweck mechanism 22 and exits the pump via outlet 30. The first interface chamber 12 is connected to a backing pump 32, which also pumps fluid from the outlet 30 of the compound vacuum pump 16. As fluid entering each pump inlet passes through a respective different number of stages before exiting from the pump, the pump 16 is able to provide the required vacuum levels in the chambers 10, 14.
FIG. 2 shows a known alternative compound pumping system suitable for use with a differentially pumped mass spectrometer. In this instance, the mass spectrometer comprises four chambers which are pumped to different pressures; a third chamber 13 is located between the first and second interface chambers 12 and 14 respectively. In this example, the vacuum pump has two pumping sections in the form of two sets 18, 20 of turbo-molecular stages, and a third pumping section in the form of a Siegbahn molecular drag mechanism 22; an alternative form of molecular drag mechanism, such as a Holweck or Gaede mechanism, could be used instead. A third pump inlet 28 connects the third chamber and fluid pumped through the inlet 28 passes through the Siegbahn mechanism or pump inter-stage 22 and exits the pump via outlet 30. Typically, the third chamber is pumped to a pressure in the transitional flow regime, between viscous and molecular flow regimes. The transitional flow regime is generally understood to be between 0.01 and 0.1 mbar.
In some such applications, a Holweck mechanism such as that illustrated in FIG. 1 typically provides a backing pressure to the second pumping section 20 of around 0.01 mbar to 0.1 mbar. The use of turbo-molecular stages for a pumping section having such a relatively high backing pressure to produce an inlet pressure of above 10−3 mbar may cause excessive heat generation within the pump and severe performance loss, and may even be detrimental to the pump's reliability. WO2006/090103 describes a compound pump comprising a helical rotor. In such a pump, during use the inlet of the helix of the helical rotor behaves like a rotor of a turbo-molecular stage, and thus provides a pumping action through both axial and radial interactions.
In some applications there is a general requirement towards higher mass throughput (gas flows) in mass spectrometer systems, so as to improve their performance. In order to increase system performance, it may be desirable to increase the mass flow rate of the sample and a carrier gas from the source into the first chamber 12, whilst maintaining a low partial pressure of neutral carrier gas in the high vacuum chamber 10. In this case, additional pumping is required at one of the intermediate chambers 13, 14 to remove the carrier gas before it reaches the high vacuum chamber 10. This can be achieved by a number of methods including the addition of more pumping stages and chambers (as shown between FIGS. 1 & 2), increasing the capacity or pumping speed of the pumping stages or increasing the conductance of the pumping ports.
For the pumps illustrated in FIG. 1 or 2, higher mass throughput could be achieved by increasing the capacity of the compound vacuum pump 16 by increasing the diameter of the rotors 21a and stators 21b of set 20. For example, in order to double the capacity of the pump 16 at the interstage between sections 20 and 18, the area of the rotors 21a and stators 21b would be required to double in size. Any molecular drag stage may also require an increase in capacity to efficiently pump molecules which have passed through the up-stream turbo-molecular stage(s). The additional volume occupied by a molecular-drag stage having increased capacity would be substantial given the relatively poor pumping capacity of such pump stages compared to turbo-molecular pump configurations. This would cause an increase in the overall size of the pump 16, and thus the overall size of the mass spectrometer system. Furthermore, increasing the pumping speed typically results in a significant increase in the pump's power consumption in non-molecular flow conditions.